1. Field of the Invention
The present invention relates to scanning antennas and more particularly to antennas capable of providing relatively high directivity with 360.degree. antenna coverage.
2. Description of the Prior Art
Many radar systems require narrow beam antennas with 360.degree. azimuthal scanning capabilities. This has been achieved with various methods one of which is the mechanical rotation of an entire antenna assembly comprising a large reflector and a feed system therefor. Due to the large inertia of these systems considerable driving power is required and the rapidity with which scanning may be accomplished is severely limited.
Mechanically scanned antennas have been developed which provide 360.degree. azimuth scan with a significantly reduced inertia than the fixed feed reflector system. In these systems the reflecting structure is a spherical or parabolic torus reflector which is stationary and the feed system is caused to rotate about the internal focal circle of the reflector. The reflecting surface comprises appropriately spaced reflecting rods which make an angle of 45.degree. with all the vertical meridians, each rod giving the effect of a barber pole stripe. By virtue of this effect a perpendicular relationship exists between rods diametrically positioned, the entire surface forming a transreflector. Thus a feed antenna positioned at the focal circle of the transreflector, radiating with a polarization vector that is parallel to reflecting rods on the inner surface of the torus which are thereby illuminated, will have a signal transmitted therefrom focussed by the illuminated area of the reflecting surface, the reflected signal appearing at the opposite surface with a polarization that is normal to the reflecting rods thereat and thereby propagate through the surface. Since the reflecting surface is circularly symmetric, the focussing of the beam is independent of the angular position of the feed antenna on the focal circle. Thus, a focussed scanned beam may be obtained by rotating the feed about the focal circle. Scanning antennas utilizing the transreflector principle are described in U.S. Pat. No. 2,835,890, issued to B. J. Bittner in May 1958 and in U.S. Pat. No. 2,989,746, issued to J. F. Ramsey in June 1961. These antennas are designed to provide beam shapes which exhibit pencil beam characteristics and though they can provide relatively rapid scanning, are limited with respect to the scan rate achievable. The present invention is directed to transreflector antennas that provide shaped beam characteristics and increased scan rates over that previously achievable.